Foaming cosmetic compositions must satisfy a number of criteria including cleansing power, foaming properties, mildness/low irritancy and physical stability. Oil-in-water emulsion systems typical of foaming cosmetic compositions represent a particular challenge with respect to stability and viscosity. While on one hand it is possible to ultilise agents specifically to induce appropriate viscosity, care must be taken to ensure these agents work to maintain that viscosity over time. Certain fatty acids are known to be effective in the achievement of matrix viscosity. However this is generally dependant upon the fatty acid forming stable complexes. It has been a particular problem that fatty acid added to achieve stability/viscosity in an oil-in-water emulsion can result in a reduction in viscosity over time. This is believed in part to be due to the secondary formation of insoluble crystalline moieties which break down the structure of the emulsion and lead to lower viscosity/thinning. The aforementioned problems can be exaggerated when products are exposed to extremes or variations of temperature.
Skin is made up of several layers of cells which coat and protect the keratin and collagen fibrous proteins that form the skeleton of its structure. The outermost of these layers, referred to as the stratum corneum, is known to be composed of 250.ANG. protein bundles surrounded by 80.ANG. thick layers. Hair similarly has a protective outer coating enclosing the hair fibre which is called the cuticle. Anionic surfactants can penetrate the stratum corneum membrane and the cuticle and, by delipidization destroy membrane integrity. This interference with skin and hair protective membranes can lead to a rough skin feel and eye irritation and may eventually permit the surfactant to interact with the keratin and hair proteins creating irritation and loss of barrier and water retention functions.
Ideal cosmetic cleansers should cleanse the skin or hair gently, without defatting and/or drying the hair and skin and without irritating the ocular mucosae or leaving skin taut after frequent use. Most lathering soaps, shower and bath products, shampoos and bars fail in this respect.
Certain synthetic surfactants are known to be mild. However, a major drawback of most mild synthetic surfactant systems when formulated for shampooing or personal cleansing is poor lather performance compared to the highest shampoo and bar soap standards. Thus, surfactants that are among the mildest, such as sodium lauryl glyceryl ether sulfonate, (AGS), are marginal in lather. The use of known high sudsing anionic surfactants with lather boosters, on the other hand, can yield acceptable lather volume and quality but at the expense of clinical skin mildness. These two facts make the surfactant selection, the lather and mildness benefit formulation process a delicate balancing act.
Despite the many years of research that have been expended by the toiletries industry on personal cleansing, the broad mass of consumers remain dissatisfied by the mildness of present day cleansing compositions, finding, for example, that they have to apply a separate cosmetic lotion or cream moisturizer to the skin after using a shower or bath preparation in order to maintain skin suppleness and hydration and to counteract the delipidizing effect of the cleanser.
Thus a need exists for personal cleansing products which produce a foam which is abundant, stable and of high quality, which are effective hair and skin cleansers, which will not dehydrate the skin or result in loss of skin suppleness, and which will provide a level of skin conditioning performance in a wash and rinse-off product which previously has only been provided by a separate post-cleansing cosmetic moisturizer, which has good rinsibility characteristics, and which at the same time has stable product and viscosity characteristics and remains fully stable under long term and stressed temperature storage conditions.